A combination of special emulsions and developers is required to give the high contrast, sharp tone, low fog and high top density characteristic of lithographic films. Such films are usually composed of one or more silver halide emulsions in hardened, macromolecular, water-permeable, organic colloid binders, deposited on a suitable support. Developers commonly used to obtain this curve shape, hereafter litho-type developers, are based mainly on hydroquinone. This combination of emulsion-developer is mainly used for the production of halftone dot images for letterpress, lithography and the like.
It is known that litho-developers require an induction period prior to development, followed by a period in which so-called "infectious development" occurs, giving rise to the high gradient necessary for good dot quality. This phenomenon is discussed by, for example, James, in the Journal of Photographic Science, Vol. 10 (1944), p. 271, and in Photographic Science and Engineering, Vol. 12 (1968), p. 67, and elsewhere.
To hasten the induction period and, hence, increase the effective speed of litho-type developer systems, and to improve developer access time, many additives have been tried. For example, it has been observed that development of exposed emulsions in mildly alkaline hydroquinone is accelerated if the emulsion is first bathed in allylthiourea. However, this results in a serious fog problem. Antifogging agents can be used to reduce this fog but they also reduce the speed of the system. Overman, in U.S. Pat. No. 3,785,822 "Photographic Emulsions and Developers Containing 2-Mercapto Heterocyclic Compounds" teaches the use of certain 2-mercapto-substituted heterocyclic compounds to increase system speed even in the presence of stabilizers and antifoggants. However, there is a need for other compounds of this type which have lower toxicity than mercaptans.